Disk arrays comprising a multiplicity of small inexpensive disk drives, such as the 51/4 A or 32/3 inch disk drives currently used in personal computers and workstations, connected in parallel have emerged as a low cost alternative to the use of single large disks for non-volatile storage of information within a computer system. The disk array appears as a single large fast disk to the host system but offers improvements in performance, reliability, power consumption and scalability over a single large magnetic disk. Several disk array alternatives are discussed in an article titled "A Case for Redundant Arrays of Inexpensive Disks (RAID)" by David A. Patterson, Garth Gibson and Randy H. Katz; University of California Report No. UCB/CSD 87/391, December 1987. The article, incorporated herein by reference, discusses disk arrays and the improvements in performance, reliability, power consumption and scalability that disk arrays provide in comparison to single large magnetic disks.
One such array system described in the incorporated article, identified as a RAID level 3 system, comprises one or more groups of N+1 disks. Within each group, N disks are used to store data, and the additional disk is utilized to store parity information. During RAID level 3 write functions, each block of data is divided into N portions for storage among the N data disks. The corresponding parity information is written to a dedicated parity disk. When data is read, all N data disks must be accessed. The parity disk is used to reconstruct information in the event of a disk failure. A RAID level 3 system including five drives is shown in FIG. 1. The disk drives are labeled DATA I through DATA 5. Data is striped across disks DATA 1 through DATA 4, each data disk receiving a portion of the data being saved. Parity information, generated through a bit-wise exclusive-OR of the data stored on drives DATA I through DATA 4, is saved on drive DATA 5.
In order to coordinate the operation of the multitude of disk drives within the array to perform read and write functions, parity generation and checking, and data restoration and reconstruction, complex storage management techniques are required. In many of the disk array systems described in the prior art, the host operates as the array controller and performs the parity generation and checking and other storage management operations. Other systems, such as the system illustrated in FIG. 1, employ a separate I/O controller associated with the disk array for controlling the operations listed above.
Currently available array controllers for RAID level 3 disk arrays utilize a buffer between the host and the array. As storage is a key determinate in system cost and performance, a transfer scheme which eliminates the requirement of a large expensive buffer between the host and the array channels is desired.